Infertility and other untoward reproductive and developmental outcomes, such as spontaneous abortion, fetal and neonatal death, birth defects, and genetic susceptibilities to cancer and other diseases, are associated with genetic damage occurring in mammalian germ cells. Test methodologies employing fluorescence in situ hybridization (FISH) to detect structural and numerical chromosome damage in sperm and early embryonic cells of rodents were developed at Lawrence Livermore National Laboratory (LLNL) under an NIEHS-DOE Interagency agreement. These methodologies are now being exported to commercial laboratories. We are participating in the use and evaluation of these test methods through the conduct of positive control experiments, by providing sperm from NTP bioassays, and by performing analyses of all data. The rodent sperm-FISH assays provide parallel models of similar sperm-FISH assays in humans. They are used for evaluating the aneugenic and clastogenic potential of environmental chemicals tested in NTP bioassays, as well as for addressing questions about dose response, differential stage sensitivity, the relationship of defects seen in sperm to those transmitted to the early embryo, and other important issues related to health-risks which cannot be addressed in human studies. During this past year, an experiment using the r-ESA on sperm collected from NOVP treated F-344 rats was designed and will be conducted in FY2002. Sperm samples from the NTP Reproductive Assessment by Continuous Breeding (RACB) contract are being frozen and retained for sperm-FISH analysis. Sperm from an RACB mouse study of AZT/ddI will be evaluated in FY2002.